The invention relates to a pneumatic tire for bicycles to be operated with an inner tube, the tire having two beads for mounting the tire on a wheel rim with rim flanges of a maximum rim flange height according to ETRTO (European Tire and Rim Trading Organization) whereby the beads have arranged therein bead cores about which a one-ply or multi-ply carcass is wound.
Especially in so-called downhill races performed with all-terrain bicycles (mountain bikes) where on downhill slopes of a great down gradient top speeds of approximately 80 km/h are reached and jumps of multiple meters are performed, it occurs, especially when the tire contact surface under extreme load is positioned on a stick or a stone, that the tire despite its great volume is compressed such that the area of the inner tube wall adjacent to the shoulder contacts the inner tube wall in the vicinity of the bead and that the inner tube thereafter experiences failure. In general, a sudden air pressure loss occurs which results in loss of maneuvering control. As a solution German Document 690 04 603 has suggested a reinforcement of the inner tube.
FIG. 1 shows in portion a known wheel for mountain bikes in cross-section of a tire 1 with inner tube 2 on a wheel rim 7. In a conventional manner the non-hose shaped tire 1 radially inwardly ends in beads 3 in which is contained a bead core 4. The center of the cross-section of the bead core 4 is indicated by reference numeral 5. About the bead cores 4 a carcass 6 is wound in a manner known per se. The anchoring of the tire 1 at the wheel rim 7 takes place, as conventional with non-hose-shaped tires, between the tire beads 3 and the rim flanges 8 in cooperation with the air pressure held by the inner tube 2. Especially for sporting applications the shown hook-shaped design of the rim flanges is widely accepted which exhibits an especially favorable ratio between construction weight and secure seating in combination with good centering properties.
FIG. 2 shows the development of the aforementioned damage in cross-section: the same wheel momentarily drives across a bump-like obstacle 13 which can be considered to be essentially stiff with respect to the stiffness of the components forming the tire equipment. In the circular areas 14 indicated in dash-dotted lines the areas of the inner tube 2 in the vicinity of the bead collide with the areas of the inner tube 2 in the vicinity of the tire shoulder. This results in considerable pressing forces between the obstacle 13 and the rim flanges 8. Due to these pressing forces the relatively soft material, conventionally a vulcanized product of a butyl rubber mixture, of the inner tube flows laterally so that the wall thickness of the inner tube at these locations is greatly reduced, during great impacts practically to zero. After overcoming the obstacle 13 upon reextension of the tire 1 a masticating movement occurs inevitably which results in failure of the inner tube at the four predamaged locations.
When the aforedescribed damage has occurred, the inner tube has four individual damaged areas arranged in a rectangle relative to one another for which reason this damage is known in the art as a snake bite.
In principle, such a damage can of course be avoided by selecting a lower speed or a different course. However, this is unacceptable to the customer, respectively, and to the race event management.
The problem could further be reduced by lowering the weight of the machine which however, in view of the high stability requirements and the minimal contribution to the total weight (weight of the rider plus bicycle), does not promise to be successful. The driver's weight is also not at one's disposal.
Furthermore, it is known to improve the energy absorption for absorbing a shock with a spring wheel suspension; however, this increases the machine weight and the handling is compromised. Furthermore, it has been tried to increase the travel of the spring by using curved instead of straight spokes between the wheel rim and the hub, especially at the front wheel. However, this impairs the fatigue resistance of the spokes and the directional stability.
Furthermore, the energy absorption can be increased by increasing the tire pressure. As a function of the softness of the ground an optimal tire pressure is selected with respect to the rolling resistance and the coefficient of friction which, depending on the ground and its moisture contents, is relatively low, i.e.; in an area between approximately 2.5 and 4.0 bar. In view of these counteracting aspects an increase of the tire pressure is hardly possible.
Physically similar problems as with mountain bikes also are present for street racing bikes despite the conventionally higher tire pressure in a range between 6 and 10 bar. This relates to the substantially smaller desired tire height which reduces the amount of rubber to be masticated and thus reduces the rolling resistance and furthermore allows for a smaller tire width which reduces the dominating air resistance effect. Due to the small tire cross-section and top speeds up to 100 km/h, pot holes however can result in a snake bite damage.
It is an object of the invention to reduce the probability of a snake bite damage without substantially increasing the rolling resistance and the weight.